The waterfall model is the oldest and the most widely used paradigm.

However, many projects rarely follow its sequential flow. This is due to the inherent problems associated with its rigid format. Namely:

1) It only incorporates iteration indirectly, thus changes may cause considerable confusion as the project progresses.

2) As The client usually only has a vague idea of exactly what is required from the software product, this WM has difficulty accommodating the natural uncertainty that exists at the beginning of the project.

3) The customer only sees a working version of the product after it has been coded. This may result in disaster if any undetected problems are precipitated to this stage.